Cnidaria

• Cnidaria have two true layers—an external layer and an internal layer—with a matrix in between.
• All Cnidaria possess cnidocytes, capsules containing a toxin-filled thread and a barbed head.
• Cnidocytes are primarily used for predation, but also for locomotion or defense.
• Tentacles are loaded with cnidocytes; sensitive hairs trigger the release of toxins.
• Hooks firmly attach to and paralyze prey.
• Cnidocytes are mainly found on the tentacles of anemones.

Cnidaria Body Forms

• Cnidaria have two main body forms, which are sometimes interchanged: polyp and medusa.
• The polyp form is typically attached to a substrate; the bulk of the body has a cavity.
• The external layer of a polyp is the ectoderm, and the internal layer is the endoderm.
• Digestion occurs in the internal cavity of the polyp.
• The medusa form is free-swimming.

Classes of Cnidaria

Anthozoa

• Anthozoa includes anemones and corals.
• They can be solitary or colonial.
• All are marine.
• The main morph type is the sessile polyp.
• Gastrocnemius cells secrete enzymes and aid digestion.
• Reproductive cells are also in the main cavity.

Myxozoa

• Myxozoa colonize, for example, the bladder of amphibians and the gills of fish.
• Spores enter the definitive hosts, i.e., other types of worms.

Medusozoa

• Medusozoa have the medusa morphotype.
• Some species combine medusa and polyp forms in development.
• They have venom in cnidocytes that paralyzes prey.

Cubozoa

• Cubozoa, e.g., box jellyfish, combines the two forms.
• The polyp transforms directly into the medusa.
• They have minute, complex eyes, a recent discovery in Cnidaria.
• They can cause economic problems by consuming fish.
• Having discovered their complex eyes, scientist now understand how they actively hunt resulting in impact of fisheries.
• They are venomous and effective hunters.

Meta Genesis

• Metagenesis is the combination of two forms in the life cycle.
• The larva colonizes a substrate and forms a polyp; then, through budding, the medusa type is formed.
• The planula is the larval type typical of cnidarians, possessing bilateral symmetry.
• All other stages of development have radial symmetry.
• Bilateral symmetry is an advantage for cephalization, allowing animals to move in one direction.

Hydrozoa

• Hydrozoa, such as hydroids, combine two forms during development.
• The adult stage consists of sessile polyps.
• They can be sessile or colonial.
• Sea fir is a British marine colonial Hydrozoa used ornamentally.
• Freshwater Hydra are present in ponds, rivers, and fish tanks and can be green or brown due to symbionts in the matrix.
• These hydra are sessile but active and can walk using tentacles.
• They reproduce sexually, with some being hermaphrodites, and all can reproduce through budding.
• One species was estimated to live for 1,400 years.
• Freshwater jellyfish, found within Hydrozoa, were imported to the UK from Brazil and are quite toxic.
• Budding is common; once fully formed, the bud detaches and swims away.
• Freshwater jellyfish are problematic as they can be transported and colonize new environments via phoresy on birds or zebra mussels.

Siphonophora

• Portuguese Man o' War are not true jellyfish and feature left and right-hand forms.
• This gigantic colonial organism has long tentacles and hunts huge numbers of fish.
• Humans can have severe reactions, including skin lacerations, convulsions, or anaphylactic shock, due to cnidocytes in the filaments.
• Predators include the sea swallow, blue angel, blue sea slug, blue dragon, and sea lizard.
• The blue dragon floats upside down, lacks its own nematocysts, and extracts cnidocytes from Portuguese Man o' War.
• Many gastropods generally predate Cnidaria.

Scyphozoa

• Scyphozoa are true jellyfish.
• Compass jellyfish are big and eat adult fish; juvenile fish sometimes hide in their tentacles for protection in a symbiotic relationship.

Corals

• Algae colonizing polyps provide oxygen and organic carbon for photosynthesis in corals.
• Warming oceans result in coral bleaching, produced by the stress of hot waters and carbon dioxide accumulation.
• The stress reacts with the calcareous structure and symbionts, causing them to leave the corals, resulting in bleaching.
• Coral reefs are located around the tropics, especially near the Equator, and are all suffering.
• 17% of Cnidaria are calcareous species, which are dissolved by carbon dioxide.
• Bio mineralization is the main source of this issue.
• Aeolid nudibranchs eat Cnidaria, hunt cnidocytes, and extract them during digestion for placement in their own tentacles.
• Anemones and clownfish share a symbiotic association, where the fish cleans the anemone.

Edibility and Symmetry

• Some types of Cnidaria are edible.
• Other entities, such as Ortiguillas in Spain, are edible.
• Phylum Ctenophora have different symmetry than Cnidaria.
• Ctenophora only have two filaments.
• Features are formed by 8 rows of ciliary plates and possess sensorial organs different from those in Cnidaria.
• Statocysts are good for location.
• Ctenophora are found very deep in the ocean.
• Furthermore, Ctenophora do not have cnidocytes; they steal them from Cnidaria.

Ctenophora

• Most Ctenophora are predators that feed on Cnidaria.
• Ctenophora have bilateral or biradial symmetry, unlike the radial symmetry of Cnidaria.

Bioluminescence

• Bioluminescence is present in both Cnidaria and Ctenophora.
• It is now used in biological surveys, microscopy, and therapies.
• GFP is extracted from Cnidaria.
• Protecting A. victoria is urgent due to its harvesting for GFP.
• GFP has a barrel-shaped structure that can be given to animals by consumption, making them visible through a microscope or scanning.